A protective coating is required to have various different properties depending on the intended application of the coating. For example, a protective coating used as a building material or building exterior material is required to exhibit good adhesion and corrosion resistance, and in some cases, a high heat resistance and capability of adhering to other substrates, and a protective coating used for an automobile should also have rust preventive properties.
The building material industry is currently experiencing a switchover from protective materials using an organic solvent to those using water in view of reducing the use of volatile organic compounds (VOC).
A protective coating formed by using a water-borne resin, however, had the problem of inferior initial water resistance compared to the protective material prepared by using an organic solvent. Accordingly, such protective coating often suffered from peeling and other problems when the protective coating experienced a high humidity condition, for example, a rainfall immediately after the coating. In view of such situation, an attempt has recently made to improve the water resistance of the water-borne protective coating by reacting acidic functional group of the anionic water-borne resin with an oxylane group-containing compound to thereby leave no hydrophilic group. This protective coating was still insufficient in its initial water resistance immediately after the coating.
U.S. Pat. No. 3,464,946 (Patent Document 1) and U.S. Pat. No. 3,444,114 (Patent Document 2) disclose an alkyl etherified amino resin which has been converted by reaction with an oxycarboxylic acid to enable its use in water-borne composition. These attempts, however, failed to solve the problem of the water resistance.
JP-B 8-32851 (Patent Document 3) discloses a two part-type water-borne coating composition which is cured with isocyanate. This composition is still insufficient in the water resistance.
Accordingly, there is a need for development of a protective coating which exhibits improved properties such as water resistance and adhesion.
In the meanwhile, a coating composition used for automobiles should have rust preventive properties. In this regard, lead salt pigments such as minium, lead cyanamide, and calcium metaplumbate, and metal chromate pigments such as basic zinc chromate and strontium chromate had once been the main reagent used for the coating composition. Use of such reagent, however, was gradually restricted in consideration of the health hazard and environmental conservation. Since then, non-polluting, non-toxic rust preventive pigments have been developed. Exemplary such rust preventive pigments include metal phosphates such as zinc phosphate, calcium magnesium phosphate, titanium phosphate, and silica phosphate; condensed metal phosphates such as aluminum tripolyphosphate; metal phosphorite such as zinc phosphorite, calcium phosphorite, strontium phosphorite, and aluminum phosphorite; zinc molybdate, calcium molybdate, barium borate, and zinc borate. These non-polluting, non-toxic pigments used in the coating composition, however, failed to exhibit rust preventive performance comparable to that of the lead salt pigments and chromate pigments, and the coating composition was equivalent to the prior art coating compositions in its use of the rust preventive pigment.
JP-A 5-140491 (Patent Document 4) discloses a water-borne etch-resistant coating composition for a metal plate produced by adding a surfactant and an acetylene alcohol compound and/or an acetylene glycol compound. In this coating composition, the uniform distribution of the resin component on the metal surface is enabled by the use of the surfactant and the acetylene alcohol compound and/or the acetylene glycol compound, and it is the resin that realizes the etch-resistance.
JP-A 2000-104015 (Patent Document 5) discloses a thermosetting coating composition comprising a water-borne polyurethane resin having the thermosetting property. Use of the urethane resin, however, is associated with the drawback of insufficient light resistance and high cost.